Development of movable faraday cup for precise charge measurement

The high brightness electron beam is required for the 8-GeV linac in SACLA in order to generate an x-ray free electron laser. The high peak current beam is generated by bunch compressors, which shorten a bunch length up to several tens of femto-seconds. The precise charge measurement of the bunched beam is indispensable for estimation of the peak current, which is important to determine the characteristics of the x-ray free-electron laser, such as a FEL gain length. The longitudinal charge distribution of the bunched beam is measured by an rf deflector. Simultaneously, the total beam charge is measured by using a current transformer (CT), which is a non-destructive beam current monitor. By using both the distribution and total charge, we can estimate the peak current value. To calibrate the current monitor, we had installed a Faraday cup, which was designed to achieve a resolution of less than 1 pC. The geometry of a collector in the Faraday cup was determined by estimations of energy loss, charge loss and thermal analyses. The collector is designed to be retractable with an actuator, in order that the beam for the user experiment is supplied without intercepting beam transport. To reduce the charge loss due to secondary emission in the collector, a bias voltage can be applied between the collector and a vacuum chamber. In order to check the performance of the Faraday cup, the beam test was carried out. An absolute accuracy of beam charge measurement of the single bunch beam with a bunch length of 20 fs or less was achieved ±2% by using the Faraday cup. The CT was calibrated with sufficient accuracy. (author)